I do all my music artwork with 3 Apps and either pictures I take myself of just nothing in particular or royalty free photos from Google. Then I use a combination of PIXLR, PAINNT (yes with two N's) and SNAPSEED. The filters in Painnt are brilliant and you can get some really good results. The cover for my peice The Oblivion Pool was an underwater photo with a few overlays and stickers added from PIXLR then run through Painnt with text added after in Snapseed. What themes are you trying to get across with your artwork..?